In recent years, from social demands for low-fuel consumption and low exhaust emission, there has been an increased attention to an electric vehicle or a hybrid vehicle which has an alternate-current (AC) motor to run. For example, in a hybrid vehicle, an AC motor is connected to a direct-current (DC) power source such as a rechargeable battery unit through a power converter such as an inverter. The inverter converts a DC voltage supplied from the DC power source to an AC voltage and drives the AC motor with the AC voltage.
A typical control system for controlling an AC motor mounted on an electric vehicle or a hybrid vehicle uses two or three current sensors to detect two or three of three phases of the AC motor. The AC motor is controlled based on outputs (i.e., current detection values) of the current sensors. In this type of motor control system, two or three current sensors are provided to one AC motor. Therefore, the size and cost of the control system may be increased.
U.S. Pat. No. 6,229,719 corresponding to JP-A-2001-145398 discloses a technique for reducing the number of current sensors in an AC motor control system, thereby reducing the cost of the AC motor control system. In the technique, one current sensor is provided to detect a current of one (e.g., U-phase) of three phases of an AC motor. A d-axis current estimation value (i.e., exciting current component estimation value) and a q-axis current estimation value (i.e., torque current component estimation value) in a rotating coordinate system of the AC motor are calculated based on the detected current and the previous current estimation values of the other phases (e.g., V-phase and W-phase). Each of the d-axis current estimation value and the q-axis current estimation value is averaged by the first-order lag filter. The present current estimation values of the other phases are calculated based on the averaged values. The AC motor is controlled by using the q-axis current estimation value (or both the d-axis current estimation value and the q-axis current estimation value).
The technique disclosed in U.S. Pat. No. 6,229,719 uses one current sensor for one AC motor, thereby reducing the size and cost of the inverter. However, depending on operation conditions, it may be difficult or impossible to ensure adequate current estimation accuracy of the AC motor. As a result, accuracy of output torque may be reduced, current control may become unstable, and torque variation may occur. Further, abnormal current or voltage may occur, and the AC motor and the inverter may be broken due to the abnormal current or voltage. Therefore, it is important to ensure adequate current estimation accuracy of the AC motor. In particular, since the electric vehicle and the hybrid vehicle have various operation conditions including a stopped condition, a high-speed running condition, a no-driving force output condition, and a rated torque output condition of the AC motor, it is very important to ensure adequate current estimation accuracy of the AC motor.